This invention relates generally to a heat and pressure, color fuser for an electrophotographic printing machine, and more particularly the invention is directed to a droop compensated fuser.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules either to a donor roll or to a latent image on the photoconductive member. The toner attracted to a donor roll is then deposited on a latent electrostatic images on a charge retentive surface which is usually a photoreceptor. The toner powder image is then transferred from the photoconductive member to a copy substrate. The toner particles are heated to permanently affix the powder image to the copy substrate.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material become tacky and coalesce. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. In a fuser where the nip is formed by the heated fuser roll, the heated fuser roll is provided with a layer or layers that are deformable by a harder pressure roll when the two rolls are pressure engaged. The length of the nip determines the dwell time or time that the toner particles remain in contact with the surface of the heated roll.
The heated fuser roll is usually the roll that contacts the toner images on a substrate such as plain paper. In any event, the roll contacting the toner images is usually provided with an adhesive (low surface energy) material for preventing toner offset to the fuser member. Three materials which are commonly used for such purposes are PFA.TM., Viton.TM. and silicone rubber.
NFFR fusers, as practiced by the industry, exhibit droop when the thermal load increases. The phenomena of droop occurs when a Nip Forming Fuser Roll (NFFR) switches from the standby mode of operation to the run mode. A large amount of thermal energy is initially removed from the heated fuser roll.
Due to thermal inertia of the fuser roll core, an internal lamp cannot prevent droop. In monochromatic (i.e. one color images only) fusers where droop takes place, the effect on copy quality is not visible or noticeable to the customer. In fusing color images, the fuser roll temperature adversely affects the appearance of the copy. Thus, the gloss and colors of color images can be adversely affected by droop. Therefore, it is desirable to maintain the fuser roll temperature at its standby value or in some other way minimize or eliminate the droop phenomena.
The object of this invention is to provide a NFFR color fuser wherein the phenomena of droop is minimized.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 4,567,349 granted to Henry et al on Jan. 28, 1986 discloses a heat and pressure fuser apparatus for fixing toner images to a substrate. The apparatus is characterized by the fact that silicone oil release agent material which is usually required for such devices is unnecessary. The fuser member which contacts the toner images comprises an outer layer of solid abhesive material capable of retaining this property without degradation over the operating life of the apparatus. The fuser member is so constructed that the abhesive coating contributes to the formation of the nip created between the fuser member and a backup roller.
U.S. Pat. No. 4,197,445 granted to Rabin Moser on Apr. 8, 1980 discloses a heat and pressure roll fusing apparatus for fixing toner images to copy substrates, the toner comprising thermoplastic resin. The apparatus includes a heated fuser roll cooperating with a backup or pressure roll to form a nip through which the copy substrates pass at relatively high (i.e. 12-20 in./sec) speeds with the images contacting the heated roll. The heated fuser roll is characterized by a relatively thick (i.e. 10 mils or greater) outer layer or surface which by way of example is fabricated from a highly insulative material such as silicone rubber or Viton to which a low viscosity polymeric release fluid is applied. Elevating the temperature of the heated roll during a standby or warm-up is accomplished by an internally disposed heating element and the operating temperature thereof during the run mode of operation is effected by an external heater.
U.S. Pat. No. 5,869,809 relates to a NFFR fuser which does not exhibit the phenomena of droop which can occur when the fuser switches from a standby to a run mode of operation. The elimination of droop is effected using an external heat source which together with an internal source of heat supplies heat to the surface of the heated fuser member to maintain its surface temperature at a preset standby value until such time as the fuser roll core reaches a temperature level sufficient to maintain the heated fuser member surface at a substantially constant temperature during standby and run modes of operation.
U.S. Pat. No. 3,861,863 discloses a black and white image fuser comprising a first stage backside heater and a second stage soft roll fuser.
U.S. Pat. No. 3,679,302 discloses first and second stage radiant fusers.
U.S. Pat. No. 3,566,076 discloses the combination of radiant and pressure fusing.
U.S. Pat. No. 4,875,611 granted Oct. 24, 1989 discloses a copy media feed system including two pairs of feed rolls which are horizontally aligned to form nip areas to engage the media. Each feed roll pair comprises one drive roll and one idler roll. For one pair, the drive roll is an elastomer-covered, high friction roll and the idler roll is a hard, roll. For the second pair the drive roll is a hard, high friction roll and the idler roll is an elastomer-covered roll. This arrangement provides accurate control of the media velocity.
U.S. Pat. No. 4,791,447 granted Dec. 13, 1988 discloses a heat and pressure fusing apparatus for fixing color toner images to various types of copy substrates. The apparatus includes three roll members which cooperate to form a pair of nips. All substrates pass through a first nip and a deflector plate directs certain types of substrates through the second nip. Passage of the substrates through the first nip causes the images carried thereon to contact a conformable elastomeric surface while passage through the second nip causes them to contact a relatively rigid surface. Thus, glossy and matte color copies on substrates such as plain paper and high chroma transparencies are suitably produced in a color reproduction apparatus incorporating this fuser. Matte color copies are produced by passing the substrate through only the first nip while glossy color copies and high chroma transparencies are produced by passing the substrates through both nips.
U.S. Pat. No. 4,639,405 granted Jan. 27, 1987 discloses a method and apparatus for fixing toner images in which a copy sheet bearing unfixed toner is first passed through a pair of heated fuser rollers and is subsequently passed through surfacing rollers to provide a gloss to the toner image. In order to prevent curling of the copy sheet and blistering of the glossed image, the copy sheet is passed through a conditioner means, located between the fuser rollers and the surfacing rollers, for removing a substantial portion of the moisture from the copy sheet.
U.S. Pat. No. 4,627,813 granted Dec. 9, 1986 discloses a thermal fixing apparatus for use with a copying machine or electronic printer in which an operating temperature of the apparatus after energization is quickly reached. A pair of fixing rolls is provided, at least one of which in heated. The outer surface of the other is covered with an elastically deformable outer layer. The two rolls are pressed into abutment with one another to form a nip therebetween of the predetermined width. A plate-shaped heater element is disposed prior to the nip adjacent the path of conveyance of toner-image-bearing paper sheets to be fixed.
The plane of the heater element is preferably parallel to the plane of the paper.
The surface temperature of the heater element has a temperature higher at central portions than at widthwise ends thereof to provide uniform fixing conditions.
U.S. Pat. No. 4,223,203 granted Sep. 16, 1980 discloses a heat and pressure fusing apparatus for fixing toner images to copy substrates comprising a first fusing system consisting of a pair of nip forming rolls, one of which is provided with a conformable outer surface and a second fusing system consisting of a pair of nip forming rolls, one of which has a rigid outer surface. Copy substrates are passed sequentially through the nips of the first and second fusing systems, in that order such that the toner images sequentially contact the conformable outer surface and then the rigid outer surface.
U.S. Pat. No. 5,521,688 granted to Rabin Moser on May 28, 1996 discloses an image treatment method and apparatus for fusing color toner images to a substrate such that they exhibit uniform gloss and satisfactory color saturation. As disclosed in this patent there is provided two fusing structures, one for partially fusing toner images on a substrate and another for completing the fusing process. The two fusing structures are arranged such that the substrate carrying toner images passes through them sequentially.